1. Field of the Invention
This invention relates generally to the field of computer graphics and, more particularly, to memory devices used in computer graphics systems.
2. Description of the Related Art
A computer system typically relies upon its graphics system for producing visual output on the computer screen or display device. Early graphics systems were only responsible for taking what the processor produced as output and displaying it on the screen. In essence, they acted as simple translators or interfaces. Modern graphics systems, however, incorporate graphics processors with a great deal of processing power. They now act more like coprocessors rather than simple translators. This change is due to the recent increase in both the complexity and amount of data being sent to the display device. For example, modern computer displays have many more pixels, greater color depth, and are able to display more complex images with higher refresh rates than earlier models. Similarly, the images displayed are now more complex and may involve advanced techniques such as anti-aliasing and texture mapping.
As a result, without considerable processing power in the graphics system, the CPU would spend a great deal of time performing graphics calculations. This could rob the computer system of the processing power needed for performing other tasks associated with program execution and thereby dramatically reduce overall system performance. With a powerful graphics system, however, when the CPU is instructed to draw a box on the screen, the CPU is freed from having to compute the position and color of each pixel. Instead, the CPU may send a request to the video card stating, “draw a box at these coordinates.” The graphics system then draws the box, freeing the processor to perform other tasks.
Generally, a graphics system in a computer is a type of video adapter that contains its own processor to boost performance levels. These processors are specialized for computing graphical transformations, so they tend to achieve better results than the general-purpose CPU used by the computer system. In addition, they free up the computer's CPU to execute other commands while the graphics system is handling graphics computations. The popularity of graphics applications, and especially multimedia applications, has made high performance graphics systems a common feature in many new computer systems. Most computer manufacturers now bundle a high performance graphics system with their computing systems.
Typically, a graphics system includes a frame buffer memory device that stores data to be output to a display. Frame buffers may be double-buffered so that data can be written into one buffer and read from the other buffer. Thus, the data may be rendered into one buffer while the data in the other buffer is being displayed. In addition to the frame buffer, some graphics systems include a texture memory device that stores texture data (e.g., groups of texels (texture elements) that make up a particular texture). Graphics systems may have additional memory needs. For example, graphics systems may need an accumulation buffer in which to store data before combining that data with other data. Some graphics systems use the system memory as an accumulation buffer. These systems may incur significant performance penalties since operations that use the accumulation buffer have to access the system memory over the system bus. In order to avoid the disadvantages of using the system memory as an accumulation buffer, some systems use a portion of the frame buffer for an accumulation buffer for rendering operations. As a result, these systems have a reduced effective frame buffer size. Additionally, these systems may have lower performance and precision for accumulation buffer operations than is desirable.